This natural history study will use a systems biology approach to identify LAIV replication niches among a variety of URT cell types and characterize the host immune response to LAIV. Healthy volunteers aged 18-49 years will be prescreened for low (<1:10) serum HAI titer against the component influenza vaccine virus strains (influenza A H1N1 and H3N2, and influenza B) of the licensed seasonal LAIV. Ten HAIlow or -negative individuals will be vaccinated intranasally with LAIV (n=8) or will receive saline intranasally (n=2). One week prior to and 2 days after vaccine administration, an NP specimen will be collected using flocked NP swabs. A blood sample will be collected at the time of NP swabbing and on Days 7 and 28 after vaccination. Total subject participation time from enrollment/baseline to the final study visit will be 5 weeks.